The provision of voice data and visual communications to mobile users has become an important area of research and product development. The degree of mobility of users varies widely, from wide area “roaming” at one end of the range, to users within a room requiring a small degree of mobility but extremely high bandwidth communications at the other end of the range. Wireless communication links are essential for providing communications with mobile users, and while radio communications are useful in providing good coverage over large areas, the data transfer rates achievable with radio communications are rather limited compared to the data transfer rates achievable with fixed networks, for instance using fibre optics. Thus radio links tend to act as a bottleneck for data. The advantage of optical links over radio links, namely providing much higher bandwidth and thus much higher data transfer rate, is well-known. Optical local area networks (LANs) have been proposed, and fall into two main types as illustrated in FIGS. 1 and 2 of the accompanying drawings. Diffuse networks, as illustrated in FIG. 1, use a wide angle source 1 and scatter the light from surfaces in the room to provide an optical “ether” similar to that which would be obtained using a local radio transmitter. This produces coverage that is robust to blocking, so that the terminal 3 can receive data despite the direct line-of-sight path being blocked by, for instance, a human being 5. However, the multiple paths between the source 1 and receiver at the terminal 3 cause dispersion of the channel, thus limiting its bandwidth. The optical transmitters required are also extremely high power, and dynamic equalisation is required for high bandwidth. Proposed networks using this approach have provided approximately 10 Mb/s “ethernet” type bandwidth. This low data rate is necessary to avoid the difficulties caused by multipath dispersion.
An alternative approach, as illustrated in FIG. 2, is to use direct line-of-sight paths between a transmitter 7 and terminals 8. These can provide much higher data rates, but the coverage area provided by a single channel can be quite small. Therefore providing area coverage and the ability of users to roam presents difficult problems. There is an additional difficulty which is that optical channels are subject to eye safety regulations, and satisfying these regulations is difficult. The eye safety regulations are particularly strict for near infrared regions (between 700 and 1400 nm), and this means that there are limits on the power of the sources. Line-of-sight networks also have the problem of blocking of the channels, by people or objects.